In mobile communication networks, there is always a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the mobile communication network is deployed.
Telecommunications standards such as the long term evolution (LTE) telecommunications standard and the high speed packet access (HSPA) standard are designed for a frequency reuse of one. This means that every network node uses the whole system bandwidth for transmission. This further means that in cellular communications systems there is no frequency planning among cells to cope with interference from neighboring cells.
Coordinated scheduling is a means to mitigate interference and improve performance in cellular communications systems. The basic idea of coordinated scheduling is to take a joint decision on when to transmit from different network nodes (such as from different eNodeBs). Examples of two network configurations to realize inter-network node coordination are centralized and distributed coordination. With centralized configuration a control unit connects a number of network nodes via e.g. optical fiber. Coordinating decisions are conducted at the control unit in a centralized manner. Ideally, coordinating the entire cellular communications system (i.e., by incorporating all cells of the cellular communications system in one single cooperating cluster) could allow for optimized performance on a complete network level. However, the complexity of coordinating a large network increases exponentially and hence global optimization may not be realistic in a real-world network environment. A traditional way to reduce the complexity of coordination is to divide the entire cellular communications system into a number of clusters by limiting the number of cells in each cluster. One way to form a cluster is to use each site as a cluster. Each cluster thus includes all the sectors of the site. A site may correspond to a network node. Each such cluster may also include micro or pico cells within the site. Such clusters have advantage of fast backhaul. However, coordinated scheduling between cells in such clusters cannot reduce inter-site interference.
With distributed configuration, the coordinating decisions may be exchanged by means of the X2 interface, or a similar proprietary-interface, between the cooperating network nodes. The network nodes that are coordinated form a so-called cooperating cluster. Scheduling decisions may then be shared within the cooperating cluster.
Traditional coordinated scheduling neither takes into account limitations in backhaul capacity and delay, nor computational resources. This makes it difficult or even impossible to implement the coordinated scheduling in practical deployments.
One particular type of traditional coordination uses direct optical fiber, for example, used in systems such as Radio over Fiber (RoF) or Remote Radio Head (RRH), to establish the cooperating cluster. The coordination is then limited to within the cluster and inter-cluster interference cannot be handled with such a configuration.
Site coordination is one alternative to achieve cooperation without relying on a direct optical fiber. However, to overcome the inter-site interference and thus to achieve performance gains, many sites and user equipment (UE) operatively connected to the sites need traditionally to be coordinated. Coordinating many sites and UEs increases the computational and signaling demands exponentially.
Standard measurement reports based upon the network characteristics may be used to define a trigger which identifies candidate UEs and cells that could benefit from coordination. One type of measurement reports available in LTE are reports from UEs receiving strong signals from a network node in a neighboring cell; these UEs transmit to the network a handover report. Handover (HO) assisted cooperation is one approach to establish cooperation without relying on direct fiber. In this approach, UEs that enter a HO zone measure strong neighboring cells, e.g. within 6 dB compare to the network node of the serving cell, and report this to the network node of the serving ce, see 3GPP 36.331, “Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Radio Resource Control (RRC)”. The coordination between the serving network node and the reported neighboring cells can be established by the X2-interface.
Further, signaling overhead increases with the number of cells in the cooperating cluster since standard measurement reports may not be sufficient for making a correct scheduling decision.
Hence, there is still a need for an improved coordinated scheduling in cellular communications systems.